The present invention relates to a contact type image sensor useful particularly for facsimile transmisson. A known form of image sensor of this type has a structure that includes a plurality of a discrete electrodes, each corresponding to a picture element to be read, which are formed on an insulated substrate. A photoelectric layer is formed over such electrodes andc thereafter a common electrode which is common to all picture elements (corresponding to all discrete electrodes) is formed over the layer. FIG. 2 shows an example of such image sensor, wherein a plurality of discrete electrodes 5 arranged in a row and a common electrode 4 overlying all of such discrete electodes are provided on opposite sides of a photoeloectric conversion layer 3, for example, of amorphous silicon (hereinafter abbreviated as a-Si) and the assembly is supported on an insulsted substrate 1.
On the occasion of reading a picture, the common electrode 4 is maintained at a fixed first potential and a signal is processed by switching sequentially the discrete electrodes 2 in turn to a second potential through the lead 5. Therefore, a single line is divided into the same number of picture elements as the discrete electrodes, and the picture elements are represented by an electrical signal extracted from the overlapped area where such discrete electrode 2 overlies the common electrode 4.
With a sensor of this kind, the number of discrete electrodes per unit length fixes at the same value the number of picture elements per unit length which can be reproduced. In some instances, it is desirable to achieve a greater number of picture elements than the number of discrete electrodes.
It is therefore an object of the present invention to provide a contact-type image sensor which permits realizing a picture eement density of two or more values almost at the same cost as the conventional type, where the picture element density is limited to a single value.